The primary objective of this collaborative project between two universities (Iowa State University or ISU and Texas Tech University or TTU) was to conduct a fundamental study to characterize, model and analyze uncertainty of tornado winds and their loading effects on buildings. The project tried to address critical gaps in the current knowledge about spatial and temporal distributions of wind near the ground level and its interaction with natural and built environments. The ISU project focused on studying (a) the interaction of tornado vortex with surface roughness and topography, (b) the effects of the horizontal motion or translation of the tornado vortex on wind speeds and their distribution near ground, and (c) wind loading effects of tornado wind on low-rise buildings based on the tornado’s characteristics and low-rise building parameters.

 The approach used in this study at ISU involved laboratory simulations using the ISU Tornado Simulator and small-scaled models of terrain, topography and low-rise buildings. A pressure-based Omni-directional velocity probe was used to map the flow field near the ground whereas pressure and force sensors were used to measure the loads on building models. The characteristics of unsteady flow structures of tornado winds and resultant wind loads on low-rise buildings were investigated in detail.

 The laboratory simulator was used to simulate a translating tornado passing over three different two-dimensional topographies: a ridge, an escarpment and a valley to assess change in: (a) the maximum horizontal wind speeds in terms of the wind amplification factor, (b) the maximum aerodynamic drag on a building in terms of the tornado speed-up ratio, and (c) the maximum duration of exposure at any location to high wind speeds of a specific range in terms of the exposure amplification factor, all with respect to a flat terrain. Results show that the maximum wind amplification factor of 14%, the maximum speed-up ratio of 14%, and the maximum exposure amplification factor of 110% corresponding to an EF3 (61–75m/s) tornado winds occur on the ridge.

 Both stationary and translating tornado-like vortices were simulated using the laboratory simulator to investigate the influence of ground roughness on their flow fields. The results show that, for the stationary tornado, the flow regime transitioned from a multi-celled core structure over a smooth terrain at a higher swirl ratio to a single or dual-celled core structure over the rough terrain. A decrease in tangential velocity and core radius, and an increase in vertical and radial velocities toward the center of the tornado were observed for both the stationary and translating tornadoes.

 Effects of translation on the near-ground tornado flow field were studied using the laboratory simulator that can physically translate over a ground plane. Two translation speeds, 0.15m/s and 0.50m/s, that scale up to those corresponding to slowly-moving tornadoes in the field were selected for this study. Compared with the flow field of a stationary tornado, the simulated tornado with translation had an influence on the spatial distribution and magnitude of the horizontal velocities, early reversal of the radial inflow, and expansion of the core radius. Maximum horizontal velocities were observed to occur behind the center of the translating tornado and on the right side of its mean path.

 The influence of tornado parameters such as swirl ratio and translation speed and a low-rise building’s spatial parameters such as its distance from the tornado mean path and its orientation with respect to the tornado’s translation direction on tornado-induced wind loads were investigated. A low-rise gable roof building (1:200 scale) with a roof angle of 35 degrees and a square plan area was chosen for this study. Laboratory simulated tornadoes with two swirl ratios with different ground-surface pressure characteristics, and three translation speeds were used. The building model was located on both sides of the simulated tornado’s mean path at several locations up to the distance of several tornado-core radii. At locations where maximum loadings occurred, orientation of the building was changed to explore its effect on peak loads. Results show significantly larger peak load coefficients for the tornado with lower swirl ratio and peak roof uplift on the building located at the tornado’s mean path to be smaller by 6-19% for the lower-swirl tornado case and up to 16% for the higher-swirl tornado case, compared to the other locations.

 Tornadoes can produce winds that can exceed 200 mph (89 m/s) causing immense property damage. This project resulted in a better understanding of underlying physics of the dynamic flow field of tornado wind and its interaction with buildings that will lead to a more accurate prediction of wind damage to buildings and improved building code in the future.  This project has raised public awareness of wind hazards and benefitted graduate, undergraduate and K-12 students, research scholars as well as public in general through improved curricula, laboratory demonstrations, student projects, publications and news media outlets.

Last Modified: 09/05/2018
Modified by: Partha P Sarkar